Correlation between highly abundant oil-prone leiosphaerid acritarchs and hydrocarbon source rocks from the Triassic Yanchang Formation, eastern Gansu Province, Northwestern China

Abstract An abundance of acritarchs, dominated by Leiosphaeridia , was extracted from the Middle and Upper Triassic cores of Ordos Basin (Northwestern China) by means of palynological maceration. Phycomata of Leiosphaeridia occurred in clusters, bunches, and single vesicular forms, occasionally with connecting macroalgal branches indicating their in-situ preservation. Apart from Proterozoic and Early Paleozoic marine Leiosphaeridia , the highly abundant leiosphaerids from the Yanchang Formation in Northwestern China likely came from a lacustrine environment, and the assemblage includes both phytoplanktonic green algae and spores of muticellular algae. TOC and δ 13 Corg in the Middle and Upper Triassic sediments of the basin show a correlation with the abundance of leiosphaerids. This correlation suggests that leiosphaerids were important biological precursors for the generation of petroleum in the main Chang 7-3 hydrocarbon source rocks.